Measure 15 inches from the floor along the wall where you would like your bed and mark a line with your level. Screw a 6-foot board at this line so it is level and 15 inches off the floor.
Hold a 33-inch board perpendicularly to the end of the 6-foot board and place two screws diagonally through the 33-inch board into the 6-foot board.
Screw another 33-inch board 22 inches from the first (perpendicular to the wall) and then 22 inches from the second, and one more on the end. All the screws should be attached diagonally in this step.
Screw the last 6-foot board to the ends of the 33-inch boards so that both 6-foot boards are parallel and all the corners are flush.
Screw one 18-inch pine board in the corner of the frame, opposite the side of the wall, and the other 18-inch board in the other corner that is opposite the side of the wall.